Jan: 4, 2015: Police investigators search for evidence after an officer-involved shooting at the San Francisco Police Mission Station in San Francisco. Officers shot and killed a man who brandished what appeared to be a handgun but was actually an air gun after they told him to leave a restricted parking lot. (AP Photo/San Francisco Chronicle, Carlos Avila Gonzalez)

SAN FRANCISCO – A man who was shot and killed by San Francisco police officers left behind several suicide notes in his cellphone, including one addressed to police, authorities said Monday.

Officers shot Matthew Hoffman, 32, Sunday evening after he entered a restricted parking lot at a police station and brandished what appeared to be a handgun. It was actually an air gun, which fires small projectiles such as pellets or BBs.

San Francisco police made public a note titled "Dear Officer(s)" with the permission of Hoffman's father, authorities said in a statement.

In the note, Hoffman, 32, said the officers "ended the life of a man who was too much of a coward to do it himself."

"Please, don't blame yourself. I used you. I took advantage of you," Hoffman added.

Hoffman was transported to San Francisco General Hospital, where he later died of his wounds. The officers were not injured.

The air gun did not have a colored tip on it, which is a standard identifier of a toy gun, Officer Gordon Shyy said Monday. He declined to discuss any other details of the case.

San Francisco Police Chief Greg Suhr said Monday that his department, like police across the nation, has heightened concern for officer safety following the ambush killings of two New York City police officers.

"This is a job where very sadly we lose officers," Suhr said. "It's on all our minds. But we've got a job to do, so we're going to do it."

The shooting occurred about 5:15 p.m. after three sergeants noticed a man loitering in the parking lot of the Mission District police station. The parking lot is clearly marked as restricted to the public, but a gate is kept open for police vehicles to come and go, Suhr said.

Police ordered Hoffman to leave, and he began to walk away but then stood in the middle of the driveway, staring at them and blocking them from leaving. The sergeants again ordered him to leave.

Hoffman began backing out of the parking lot, facing the sergeants with his hands in his front shirt pockets. They asked him to show his hands. He lifted his sweater and showed what appeared to be the butt of a handgun, Esparza said.

When Hoffman reached for his waistband and pulled out the air gun, two of the sergeants opened fire, hitting Hoffman three times.

Hoffman had approached officers at the Mission District station earlier in the day and asked them what kind of guns San Francisco police carry, what kind of ammunition they use and if the officers had been involved in any officer-involved shootings, Esparza said.

The officers did not speak with the man, and he eventually left.

The sergeants involved in the shooting were placed on 10 days of paid administrative leave, which is standard in any police shooting. Their names were not released.

Suhr said he will hold a town hall meeting discussing the shooting, which rattled nearby residents.

Joann Kuhl, who lives a few blocks away, told the San Francisco Chronicle that her biggest concern was that the shooting might "fuel the fire of a really bad state of affairs between police and their community."